An actuator system for hydraulic actuation of a clutch includes a hydrostatic actuator, as known for example from German Patent Application Nos. DE 10 2010 047 800 A1 or DE 10 2010 047 801 A1. An actuator of this sort actuates a master piston in a master cylinder, in order to exchange a hydraulic fluid with a slave cylinder by means of a pressure line. The slave cylinder includes a slave piston, which actuates the clutch.
In one embodiment, two clutches are provided in the drivetrain, which operate on different transmission shafts, with only one of the clutches being engaged at a time. It is usual here that the clutch transmits power into the drivetrain when the pressure in the slave cylinder is sufficiently high. An inverted embodiment, in which a clutch transmits power into the drivetrain when the pressure in the slave cylinder is low, is known from a clutch system that has just one clutch.
A position of the master piston is coupled to the position of the clutch, but this coupling is subject to a number of influencing factors. For example, the clutch system, in particular the hydraulic fluid, may heat up during operation, which causes it to expand, or a pressure of the hydraulic fluid increases. Conversely, a cooling of the clutch system may result in a reduced volume or pressure of the fluid. In order to balance out such effects, it is usual to periodically connect the hydraulic fluid, which is closed off in the region between the pistons, to a readjustment container by means of an equalizing opening (“sniffing”), in order to balance out underpressure or overpressure. To that end, a connection opening is provided between the readjustment container and the master cylinder, which is released when the master piston is in the so-called sniffing position. However, as the connection opening is closed from the sniffing position, the hydraulic pressure already rises before the connection opening is completely closed. During this phase, a quantity of hydraulic fluid is exchanged between the readjustment container and the master cylinder, which depends on a flow resistance of the hydraulic fluid. However, the flow resistance is dependent on the temperature of the fluid, among other things, so that after the connection opening is closed a predetermined volume or predetermined pressure of the hydraulic fluid in the actuator system cannot be guaranteed.